Pandey

BASANTA PANDEY LTE-3G INTER-OPERABILITY STUDY Master of Science Thesis

Topic approved by: Faculty Council of

Electrical Engineering on 6th February 2013.

Examiners:

Professor, Dr. Tech. Mikko Valkama Dr. Tech. Jarno Niemel

I

ABSTRACT TAMPERE UNIVERSITY OF TECHNOLOGY

Master's Degree Programme in Electrical Engineering

PANDEY, BASANTA: LTE-3G INTER-OPERABILITY STUDY

Master of Science Thesis, 95 pages. 2 Appendix pages

October 2013

Major: Radio Frequency Electronics

Examiner(s): Prof., Dr. Tech. Mikko Valkama

Dr. Tech. Jarno Niemel

Keywords: LTE, 3G, mobility, inter-RAT handover, KPIs

In this thesis the author has studied and measured how LTE Release 8 interworks with

previous legacy 3G networks in real environmental conditions. At present, LTE tech-

nology is deployed based on service hotspots that cover small geographical areas. It is

expected that full scale deployment of LTE network will take a considerable time,

which also means the mobile users have to primarily depend on legacy 3G and 2G net-

works for years to come. Therefore, it is important to study the interworking mecha-

nisms between LTE and legacy networks in order to provide seamless mobility and un-

interrupted user services in primarily available LTE hotspots.

In order to perform this study, field measurements have been carried out in DNA com-

mercial network in outdoor and indoor environments. Initially, cell selection and rese-

lection criteria for inter-RAT mobility in idle condition is mathematically checked and

verified. Then, channel conditions are studied and analyzed based on radio parameters

like RSRP, RSCP, RSRQ, Ec/No, SNR and CQI when inter-RAT handover is per-

formed. After that, an inter-RAT handover test from LTE towards 3G is studied with the

help of signalling message. Next, the impact of inter-RAT handover on KPIs like MAC

DL throughput, handover success rate, RTT, handover latency and user plane delay are

studied and analyzed. Finally, performance of inter-RAT handover in outdoor and in-

door measurement environment is compared based on KPI measurements.

From this study, it is found that inter-RAT mobility from LTE towards 3G network is

working in both idle and connected modes with 100 percent handover success rate,

however, the user experienced network latency around 4 seconds in average. The user

experienced degradation in throughput because of decreasing link quality. The user data

service interruption is roughly for 3-4 seconds and the RTT value for 32 bytes of data is

observed to be around 300 ms in average during handover. It is also found that the im-

pact of inter-RAT handover in indoor environment is higher than outdoor environment

based on KPIs results.

II

PREFACE

This Master of Science Thesis has been written for the completion of Master of Science

Degree in Electrical Engineering from the Tampere University of Technology, Tampe-

re, Finland. The thesis work has been carried out in the Department of Electrical Engi-

neering under Radio Network Planning Group during the year 2012 and 2013.

I would like to thank my examiner Professor, Dr. Tech. Mikko Valkama and supervisor

Dr. Tech. Jarno Niemel for supervising and guiding me throughout my thesis work. I

would also like to thank Tero Isotalo and Professor Jukka Lempiinen for their continu-

ous guidance and support during thesis. I am extremely grateful to my supervisor Jarno

Niemel for helping me in drive test during the measurement. Without this, completion

of thesis would not be possible. Thanks to all my colleagues in Radio Network Group

for their friendly behaviour and support during this thesis.

Finally I would like to express my gratitude to my family members for their continuous

encouragement throughout my studies.

Tampere, 11th October, 2013

Basanta Pandey

[email protected]

III

TABLE OF CONTENTS

1. INTRODUCTION .................................................................................................... 1

1.1 Objectives and Limit of the Research ............................................................ 1

1.2 Research Methods .......................................................................................... 2

1.3 Thesis Structure .............................................................................................. 2

2. MOBILE COMMUNICATION SYSTEM ............................................................... 3

2.1 Cellular Concept ............................................................................................. 3

2.2 Location Management .................................................................................... 5

2.2.1 Location Area (LA) .......................................................................... 5

2.2.2 Routing Area (RA) ........................................................................... 5

2.2.3 Tracking Area (TA) ......................................................................... 5

2.3 Handovers....................................................................................................... 6

2.4 Radio Propagation Environment .................................................................... 6

2.5 Radio Channel Properties ............................................................................... 7

2.5.1 Multipath Propagation and Delay Spread ........................................ 7

2.5.2 Angular Spread ................................................................................ 8

2.5.3 Fast fading and Slow fading............................................................. 8

2.5.4 Propagation Slope ............................................................................ 9

2.5.5 Characteristics of Radio Propagation Environments ..................... 10

2.5.6 Propagation Path Loss Models ....................................................... 10

2.6 Multiple Access Schemes............................................................................. 12

3. HISTORY AND LTE OVERVIEW ....................................................................... 14

3.1 History of Mobile Networks ........................................................................ 14

3.1.1 Evolution towards 1G .................................................................... 14




3.2 Overview of UMTS System ......................................................................... 16

3.2.1 UMTS Network Architecture ......................................................... 17

3.2.2 UMTS Physical, Transport and Logical channels ......................... 18

3.3 High Speed Downlink Packet Data Access (HSDPA) ................................. 20

3.4 LTE Evolution and Upgrade Path ................................................................ 20

3.5 LTE Network Architecture........................................................................... 21

3.5.1 User Equipment (UE)..................................................................... 22

3.5.2 Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) . 22

3.5.3 Evolved Packet Core (EPC) ........................................................... 23

4. LTE RADIO INTERFACE ..................................................................................... 25

4.1 Air Interface Technologies for LTE ............................................................. 25

4.1.1 OFDMA for Downlink Transmission ............................................ 25

4.1.2 SC-FDMA for Uplink Transmission.............................................. 27

IV

4.1.3 Multiple Antenna Technology ....................................................... 28

4.2 LTE Framing Structure ................................................................................ 29

4.3 LTE Interface and Protocols ........................................................................ 30

4.4 LTE Physical, Transport and Logical Channels........................................... 32

4.4.1 Physical channels and signals ........................................................ 33

4.4.2 Transport Channels ........................................................................ 34

4.4.3 Logical Channels............................................................................ 35

4.5 Scheduling .................................................................................................... 36

4.6 Link Adaptation............................................................................................ 36

4.7 HARQ........................................................................................................... 36

4.8 Power Control .............................................................................................. 37

4.9 RRM Functions ............................................................................................ 37

5. MOBILITY IN LTE ................................................................................................ 39

5.1 EPS Mobility and Connection Management ................................................ 39

5.1.1 EPS Connection Management (ECM) ........................................... 39

5.1.2 EPS Mobility Management (EMM) ............................................... 40

5.2 Mobility Management in Idle Mode ............................................................ 40

5.2.1 Public Land Mobile Network (PLMN) selection ........................... 40

5.2.2 Cell Selection ................................................................................. 41

5.2.3 Cell Re-selection ............................................................................ 42

5.2.4 Example of inter-RAT cell selection and reselection .................... 44

5.3 System Information ...................................................................................... 46

5.3.1 System Information Block 1 (SIB1) .............................................. 47

5.3.2 System Information Block 3 (SIB3) .............................................. 47

5.4 Mobility Management in Connected Mode ................................................. 48

5.5 Hard Handover in LTE ................................................................................. 48

5.5.1 X2 based Handover ........................................................................ 48

5.5.2 S1 based Handover......................................................................... 49

5.6 Inter Radio Access Technology Handovers ................................................. 50

5.6.1 Handover from EUTRAN to UTRAN ........................................... 50

5.6.2 Handover from UTRAN to EUTRAN ........................................... 54

5.7 Measurement Events and Triggering ........................................................... 57

5.8 VoLTE and SR-VCC ................................................................................... 64

6. MEASUREMENT AND RESULTS ...................................................................... 66

6.1 Performance Parameters ............................................................................... 66

6.1.1 UMTS User Equipment Measurements ......................................... 66

6.1.2 LTE User Equipment Measurements ............................................. 67

6.1.3 Downlink Throughput .................................................................... 67

6.1.4 Link Adaptation ............................................................................. 68

6.1.5 Handover Success Rate .................................................................. 69

6.1.6 Control Plane Latency .................................................................... 70

6.1.7 User Plane Latency ........................................................................ 71

V

6.2 Measurement and Post-processing Tools ..................................................... 72

6.3 Measurement Campaigns ............................................................................. 73

6.4 Measurement Setup ...................................................................................... 74

6.4.1 Outdoor Scenario ........................................................................... 74

6.4.2 Indoor Scenario .............................................................................. 75

6.5 Measurement Results ................................................................................... 75

6.5.1 Connected Mode Mobility ............................................................. 76

6.5.2 Channel Condition Comparison ..................................................... 77

6.5.3 MAC DL Throughput .................................................................... 81

6.5.4 Handover Success Rate .................................................................. 84

6.5.5 Control Plane Latency .................................................................... 85

6.5.6 User Plane Latency ........................................................................ 86

7. CONCLUSIONS AND DISCUSSION .................................................................. 90

REFERENCES ................................................................................................................ 92

APPENDIX ..................................................................................................................... 96

VI

LIST OF ABBREVIATIONS 1G First Generation

2G Second Generations

3G Third Generations

3GPP Third Generation Partnership Project

4G Fourth Generations

AM Acknowledge Mode

AF Application Function

AMPS Advanced Mobile Phone System

APN Access Point Name

AuC Authentication Centre

ATM Asynchronous Transfer Mode

AT&T American Telephone & Telegraph

BCH Broadcast Channel

BCCH Broadcast Control Channel

BSS Base Station Subsystem

CA Carrier Aggregation

CDF Cumulative Distribution Function

CN Core Network

CP Control Plane

CP Cyclic Prefix

CPICH Common Pilot Channel

CQI Channel Quality Indicator

DCCH Dedicated Control Channel

DFT Discrete Fourier Transform

DHCP Dynamic Host Control Protocol

DL Downlink

DLSCH Downlink Shared Channel

DSP Digital Signal Processing

DTCH Dedicated

EDGE Enhanced Data rates for GSM Evolution

EIR Equipment Identity Register

EMM EPS Mobility Management

eNodeB Evolved NodeB

EPC Evolved packet Core

EPS Evolved Packet System

EUTRAN Evolved UTRAN

FACH Forward Access Channel

FDM Frequency Division Multiplexing

VII

GGSN Gateway GPRS Support Node

GI Guard Interval

GPRS General Packet Radio Service

GSM Global System for Mobile Communications

GTP GPRS Tunnelling Protocol

HLR Home Location Register

HO Handover

HSDPA High Speed Downlink Packet Access

HS-DSCH High Speed Downlink Shared Channel

HSPA High Speed Packet Access

HSS Home Subscription Server

HSUPA High Speed Uplink Packet Access

HTTP Hypertext Transfer Protocol

iDEN integrated Digital Enhanced Network

IETF Internet Engineering Task Force

IFFT Inverse Fast Fourier Transform

IMEI International Mobile Equipment Identity

IMSI International Mobile Subscriber Identity

IP Internet Protocol

IS Interim-Standard

ISDN Integrated Services Digital Network

ISI Inter Symbol Interference

J-TACS Japanese Total Access Communication System

KPI Key Performance Indicator

LTE Long Term Evolution

MAC Medium Access Control

MCH Multicast Channel

MCCH Multicast Control Channel

MIB Master Information Block

MIMO Multiple Input Multiple Output

MISO Multiple Input Single Output

MME Mobility Management Entity

MMS Multimedia Message Service

MSISDN Mobile Station Integrated Service Digital Network

MTCH Multicast Traffic Channel

NAS Non-Access-Stratum

NMT Nordic Mobile Telephone System

NSS Network Switching Subsystem

NTT Nippon Telephone and Telephone Company

O&M Operation & Maintenance

OFDMA Orthogonal Frequency Division Multiple Access

OLPC Open Loop Power Control

VIII

PAPR Peak to Average Power Ratio

PBCH Physical Broadcast Channel

PCC Policy and Charging Control

PCH Physical Control Channel

PCCH Paging Control Channel

PCFICH Physical Control Indicator Channel

PCI Physical Cell Identity

PCRF Policy and Charging Resource Function

PDCCH Physical Data Convergence Protocol

PDCP Packet Data Convergence Protocol

PDC Personal Digital Cellular Technology

PDCCH Physical Downlink Control Channel

PDSCH Physical Downlink Shared Channel

PGW Packet Data Network Gateway

PHY Physical Layer

PHICH Physical Hybrid ARQ Indicator Channel

PLMN Public Land Mobile Network

PMI Precoding Matrix Indicator

PRACH Physical Random Access Channel

PRB Physical Resource Block

PSTN Public Switched Telephone Network

PUCCH Physical Uplink Control Channel

PUSCH Physical Uplink Shared Channel

QAM Quadrature Amplitude Modulation

QoS Quality of Service

QPSK Quadrature Phase Shift Keying

RACH Random Access Channel

RAT Radio Access Technology

RLC Radio Link Control

RNC Radio Network Controller

RRC Radio Resource Connection

RRM Radio Resource Management

RSCP Received Signal Code Power

RSRP Reference Signal Received Power

RSRQ Received Signal Received Quality

RTT Round Trip Time

SC-FDMA Single Carrier Frequency Division Multiple Access

SCTP Stream Control Transfer Protocol

SFBC Space Frequency Block Code

SGSN Serving GPRS Support Node

SGW Serving Gateway

SISO Single Input Single Output

IX

SIMO Single Input Multiple Output

SMS Short Message Service

SMPT Simple Mail Transfer Protocol

SNR Signal to Noise Ratio

SR-VCC Single Radio Voice Call Continuity

TA Tracking Area

TACS Total Access Communication System

TAI Tracking Area Identity

TAL Tracking Area List

TAU Tracking Area Update

TDMA Time Division Multiple Access

TM Transparent Mode

TTI Transmission Time Interval

TTY Tampereen Teknillien Yliopisto

UE User Equipment

UM Unacknowledged Mode

UMTS Universal Mobile Telecommunication System

UP User Plane

USIM Universal Subscriber Identity Module

USCH Uplink Shared Channel

UTRAN Universal Terrestrial Radio Access Network

WCDMA Wideband Code Division Multiple Access

VAS Value Added Service

VLR Visitor Location Register

VMS Voice Message Service

VOLTE Voice over LTE

X2AP X2 Application Protocol

X

LIST OF SYMBOLS

Srxlev Cell selection received signal level

Ec/No Energy per chip divided by power density of the band

Qrxlevmeas Measured RSRP value

Qrxlevmin Required minimum RSRP value

Qrxlevminoffset Offset to signalled Qrxlevmin

Pcompensation Power compensation

Sservingcell Serving cell measured value

Sintrasearch Intra-frequency cell selection search threshold

Snonintrasearch Inter-frequency cell selection search threshold

Qmeas,s Serving cell measured value

Qmeas,n Neighbouring cell measured value

Qhys Hysteresis value

Qoffset Offset value

Treselection Time to trigger

PEMAX maximum allowed uplink transmit power within a cell

PUMAX maximum transmit power capability of the UE

Rn Rank of the neighbour cell

Rs Rank of the serving cell

1

1. INTRODUCTION

Technology with low cost and offering high quality of service are always on user's

choice. Several efficient technologies are developed and implemented by the mobile

operators to satisfy mobile users. In order to meet the expectation of high mobile data

rate, quality of service, faster communication and facilitating multimedia service; High

Speed Packet Access (HSPA) technology based on the Universal Mobile Telecommuni-

cation System (UMTS) called 3G networks is introduced and deployed. This technology

has made drastic change in raising the numbers of mobile data service users day by day

at exponential rate. As this growth continuous in future, the 3G network might not be

enough to hold the network load. Therefore, a new technology providing even more

capacity and high data rates than 3G network is developed by 3rd Generation Partner-

ship Project (3GPP) called Long Term Evolution (LTE) to fulfil the requirement.

LTE is a 4th generation (4G) wireless network based on packet switched technique with

flat architecture. This architecture is able to provide high data rates, lower latencies,

high spectral efficiency and compatible with previous 3GPP networks like UMTS and

Global System for Mobile Communication (GSM) as well as non-3GPP networks.

Users always expect uninterrupted, efficient and stable service from mobile network

while moving from one place to another. Mobility supports user to move from one place

to another without breakdown of ongoing service within coverage area. Mobility of a

user is controlled by the handovers algorithms. These algorithms are developed to en-

sure the consistent performance of the cellular network to offer seamless mobility such

that user quality of service is always maintained.

1.1 Objectives and Limit of the Research

The goal of this thesis is to study and perform the test measurements regarding LTE

interworking with previous legacy 3G networks. Measurement should be done in realis-

tic radio conditions and test the actual mobility to see the impact on different KPIs for

commercial network operators like DNA and Elisa in Finland.

The main outcome of this thesis is the analysis of inter-RAT mobility testing in DNA

network from LTE to 3G network in Tampere University of Technology (TUT) campus

region in indoor and outdoor measurement environments. The analysed results in this

thesis are for the connected mode inter-RAT mobility with a data connection experi-

enced by a single user.

The main limitation is the mobility from 3G towards LTE in this thesis. This is because

of wide coverage of 3G networks including LTE hotspot areas. But theoretical explana-

tion of inter-RAT handover from 3G to LTE process is given in detail. One major diffi-

culty during this thesis research is finding the LTE to 3G inter-RAT handover spots

during mobility.

2

1.2 Research Methods

This thesis starts from the literature study from different books, technical papers, con-

ference papers, journal documents and different websites. In the beginning the neces-

sary background theory is provided in simplest manner to help the reader to understand

the basic concept, terms and the theories used while analyzing the measured results. The

relevant documents used are enlisted under reference headings. Before starting actual

measurement, two other measurements were performed: one on intra-cell mobility in

LTE within Nokia Test Network in Tampere University of Technology (TUT) premises

and other inter-Frequency mobility in LTE between indoor test network of TUT and test

network of Nokia in Hermia region. These are done for background information and

understand the basic concept of handover signalling message flow between the network

elements. After that inter-RAT mobility measurement is conducted in two locations:

outdoor and indoor. Both measurements are done in Tampere University of Technology

(TUT) campus area. Nemo Outdoor is used to monitor and record the measurement

while Nemo Analyzer is used for analyzing the data. The data is extracted and filtered

with the help of MS-Excel and the required data is plotted in the Matlab for results. Fi-

nally analysis and conclusion is made based on these results.

1.3 Thesis Structure

The entire thesis is divided into seven chapters. Chapter 1 introduces the topic, goals

and limitation and the research methodology used in this thesis. Chapter 2 explains the

cellular concept, location management schemes used in mobile communication and dif-

ferent types of radio propagation environments. It also deals with the properties of radio

channel and different types of channel accessing schemes used in mobile technology.

Chapter 3 gives a brief discussion on LTE. It also explains history about the mobile

network and evolution path towards LTE. It also gives a detail description on network

elements associated with UMTS and LTE network architecture. Chapter 4 focuses on

the air interface technology used in LTE. The framing concept, interface and protocols

and the different channels used are discussed under same chapter. It explains about the

Medium Access Control (MAC) layer and physical layer functions as well. Chapter 5 is

the core chapters of literature for this thesis to understand the analysis and results.

Chapter 5 begins with a short introduction on system information message and the

measurement events in LTE. After that, it gives a detail description on the mobility

management within LTE and with previous legacy 3G networks in both idle and con-

nected mode with examples. At last this chapter ends by giving a short introduction on

Voice over LTE (VoLTE) and Single Radio Voice Call Continuity (SR-VCC). Chapter

6 begins with the discussion on different key performance indicators used to evaluate

the network performance and gives detail information about the measurement and ana-

lyzing tools. This chapter ends with measurement results and discussions. Finally Chap-

ter 7 concludes the overall thesis.

3

2. MOBILE COMMUNICATION SYSTEM

Communication system helps to exchange the information between a sender and a re-

ceiver. The communication process becomes effective only when a receiver understand

the exact information sent by a sender. A sender and a receiver are always connected

with each other by means of communication medium. These communication medium

may be guided lines or wireless. In guided lines, the information is guided along a phys-

ical path directly connected between a sender and a receiver. Twisted pair cable, coaxial

cable and optical fibers are some examples of guided lines. In wireless, the information

propagates in the form of electromagnetic waves through air. Microwaves and radio

waves are examples of wireless media. Mobile communication, a wireless technology

allows a sender or a receiver to communicate with each other anytime, anywhere and

anyone.

This chapter starts with the introduction on cellular concept. It contains the detail de-

scription about different radio propagation environment and its channel properties. It

also explains about the different access technologies used in wireless communication. A

short concept behind the spread spectrum technology is explained at the end of this

chapter.

2.1 Cellular Concept

The idea of the cellular concept was proposed by Bell Labs (AT&T) in 1947. The main

aim behind the development of this concept was to use the available spectrum in effi-

cient manner using low power transmitters and to provide full coverage with high ca-

pacity and to ensure full mobility within coverage area with uninterrupted service.

In this concept, a large geometrical area is divided into smaller areas called cells and

these cells are grouped together to form a cluster. These sub divided areas utilized fre-

quency reuse mechanism. In this mechanism, the radio frequency or radio channel used

by a cell can be utilized by another cell after a certain physical distance called reuse

distance. It means the radio channels cannot be used in adjacent neighbouring cells.

This is done to avoid co-channel interference. This frequent use of radio resources in-

creases the capacity. The reuse distance, D is calculated by: [1]

3D R N (2.1)

where R is the radius of the cell and N is the number of cells per cluster. The valid

cluster size can be constructed if: 2 2*N i i j j (2.2)

where i and j are non negative integers and is given as: 0i and j i .

4

The shape of the cells can be square, circular, and hexagonal or some other irregular

shapes. The shape is chosen in such a way that it should be geometrical, cover the areas

without overlap or leave no gaps and has the largest area. The hexagonal shape is the

best that satisfies all these conditions. So, it is universally adopted. Figure 2.1 is an ex-

ample of hexagonal cellular concept with frequency reuse factor 3. Here a geometrical

area is subdivided into small cells. Each cell is separated by different colours using

three different frequency f1, f2 and f3 after certain reuse distance.

Figure 2.1: Hexagonal cell with frequency reuse factor 3

The different types of cell size are deployed depending upon the coverage and capacity.

Macro cells: These types of cells are deployed to cover remote and sparsely

populated areas. They cover around 10km or even more.

Micro cells: They cover around 1km in diameter and mostly deployed in densely

populated areas.

Pico cells: They are deployed to cover very small areas like buildings and offic-

es or to such types of place where the coverage from the large cell is not possi-

ble.

As the height of the base station decreases, the size of the cell becomes smaller. Smaller

cell radius requires smaller transmit powers. This helps to reduce the mobile battery

consumption. More number of base stations can be added to increase radio capacity

from fixed radio spectrum to serve more users. The cellular concept has some draw-

backs. To increase the capacity, more number of base stations is required. This increases

the cost. It should also support seamless handoff between the cells as radio channel

condition varies throughout the network. Management of the resource is required and

need to track the user location to route incoming call/message.

Those channels assigned to a cell are classified as downlink channels and uplink chan-

nels. Downlink channels are used to carry traffic from the base station to mobile stations

whereas uplink channels are used to carry traffic from mobile stations to the base sta-

tion. These channels are further divided into control channels and traffic channels. Con-

trol channels carry control information while the traffic channels carry user voice or

data information. A mobile station communicates another mobile station via base sta-

tion. At first, the network needs to know the location of the target mobile station in a

cell before starting communication.

f1

f1

f1

f2

f2

f2

f3

f3

f3

5

2.2 Location Management

In cellular network Location Management (LM) tracks the location of an active mobile

station. A mobile is said to be active if it is powered on. The LM involves two opera-

tions: location update and paging.

Paging: Paging is always performed by cellular network. The cellular network

will page in all possible cells to find out the cell in which the active mobile sta-

tion is located.

Location update: This operation is always performed by the active mobile sta-

tion. This is done either by globally or locally. A global location update scheme

allows all mobile stations to update their locations at the same set of cells

whereas local location update allows each mobile user to decide when and where

to perform location update.

2.2.1 Location Area (LA)

This is an approach for location management used in first generation and second gen-

eration system like Global System for Mobile Communication (GSM). A serving area is

subdivided into location areas and each location area consists of several adjacent cells.

The base station of each cell broadcast the identification of the location area which is

called Location Area Identity (LAI) to which the cell belongs. A mobile station updates

its location area and informs the cellular network by sending Location Area Update

(LAU) code whenever it enters into a cell which belongs to a new location area.

2.2.2 Routing Area (RA)

This approach is used for location management in second generation and third genera-

tion system like Universal Mobile Telecommunication System (UMTS). The routing

area is sub-area of a location area with specific means for PS services. Each user in-

forms Serving GPRS Support Node (SGSN) about RA to which the user resides. Each

RA has its own Routing Area Identity (RAI) and is updated when a mobile station's

routing area is changed. This RAI consists of Location Area Code (LAC) and RAC

message.

2.2.3 Tracking Area (TA)

This location management approach is used for Evolved Universal Terrestrial Radio

Access Network (E-UTRAN) cells. They are used to track the mobile stations which are

in standby mode. Adjacent EUTRAN cells are grouped together to form tracking area.

These grouped cells have same Tracking Area Identity (TAI). TAI may vary when the

mobile station moves from one cell to another cell. The mobile station reports TAI up-

dates by sending Tracking Area Update (TAU) message. The number of cells to be

paged to find the location of the cell depends on tracking area. Signalling overhead may

6

rise if frequent TAU happens. Therefore a concept of Tracking Area List (TAL) is in-

troduced. In this concept, each cell belongs to only one TA but a mobile station can be

registered to many TAs at the same time. These TAL is formed by the collection of TAs

to which a single UE is registered.

2.3 Handovers

Handover maintains a connection between the network and the Mobile Station (MS)

when MS moves from one cell to another. As the mobile station moves towards the cell

edge the signal strength starts to deteriorate. Therefore the MS has to find the new cell

and camped into it before ongoing services session gets disturbed. Therefore handover

process helps to find a suitable cell for UE to maintain quality of service. Generally,

handover may be hard, soft and softer. Hard handover is the 'break-before-make' hando-

ver. In this type, the channel of the source cell is released before connecting to the

channel of the target cell. Soft handover is the 'make-before-break' handover in which

the channel of the source cell is released only after connecting to the target cell. Softer

handover is a type of soft handover the radio channels that are connected and released

belong to the same site.

The handover process comprises three steps. The first step is the handover initiation.

Either MS or network initiates the handover once needed. The second stage is the new

connection establishment by finding the available resources for handover process and

routing operations. Last stage is the successful data flow from the new established con-

nectivity. Handover may be intra-cell, inter-cell and inter-RAT cell. Intra cell occurs

when the MS moves within a serving area of the same network. Inter cell handover oc-

curs when the UE moves into adjacent cell within a network and inter-RAT handovers

occurs when the MS moves from one technology to another. Refer Chapter 5 for detail.

2.4 Radio Propagation Environment

The transmitted radio waves from transmitter and receiver depend upon the environ-

ment on which it propagates. The propagation paths between them vary the performance

of the communication system. Therefore the coverage and the capacity of any cellular

systems rely on the behaviour of different environment. The classification of the envi-

ronment is done to make the network planning process easier. Figure 2.2 is the classifi-

cation of the environment in terms of: [2]

Mobile location: The mobile terminal outside the buildings environment is

termed as outdoor whereas mobile located inside is termed as indoor.

Antenna location: Depending upon the location of antenna, the environment is

classified as macro, micro and pico. In macro-cellular, the location of the anten-

na is above the average height of the buildings whereas in micro-cellular the an-

tenna is below the rooftop level. In pico-cellular the antennas are completely lo-

cated inside the buildings.

7

Morphography type: The environment is classified into urban, suburban and ru-

ral depending upon the population density and natural obstacles present in the

surroundings. Urban areas are characterized by the high population densities like

cities or towns. Suburban area may be a part of city with low population density

compared to urban type. Rural areas are village areas located outside the cities

and have least population density.

Figure 2.2: Classification of radio propagation environments

2.5 Radio Channel Properties

This section describes the parameters that characterized the propagation environment.

2.5.1 Multipath Propagation and Delay Spread

In mobile communication, the signal propagation path between the mobile station and

base station is affected by the objects/obstructions present in between them. These ob-

stacles cause the transmitted signal to reflect, diffract and scatter. Reflection is caused

by the wall of the buildings and earth surface. Sharp edge of walls, mountains and roof-

tops cause diffraction of the signal and trees are the source of scattering the signal. Due

to these obstacles the received signal consists of several replicas of the originally trans-

mitted signal. These replicas have different amplitudes, phase, polarisation and angle of

arrival. This phenomenon of transmitted signal arriving from different path at the re-

ceiver is called multipath propagation.

The multipath propagation caused the signal to arrive at different time instants. The

variation of this timing instant is measured by delay spread. The delay spread S is cal-

culated from power delay profile (PDP) P . [2]

Micro celluar

Urban

Suburban

Rural

Propagation Environment

Indoor

Outdoor

Macro cellular Pico cellular

8

2

0

* *

total

P d

SP

(2.3)

where power delay profile is power of the received signal received at different time in-

terval through multipath.

is the average delay and totalP is the total received power.

Different propagation environment has different delay spread. The macro cellular envi-

ronment has high delay spread than in micro cellular and indoor environment due to

high arrival time of multipath component.

Frequency separation of the multipath components is given by the coherence bandwidth

cf which depends upon the delay spread. Coherence bandwidth is range of frequencies

over which a channel is considered flat. The relation between the coherence bandwidth

and delay spread is given by,

1

2cf

s

(2.4)

where s is the delay spread. [2]

2.5.2 Angular Spread

The variation of the signal incident angle of the received power due to multipath is giv-

en by angular spread ( S ). It can be calculated either in horizontal or vertical planes

using formula,

2180

180 total

PS d

P

(2.5)

where is the incident angle, is the mean angle, P is the angular power distri-bution and

totalP is the total power. The angular spread from the horizontal planes is

mostly concerned. This is due to large amount of propagation paths between the mobile

station and base station. The horizontal angular spread is high in indoor and micro cellu-

lar environment than in macro cellular environment. In indoor environment, it has 360

degrees of variation; in micro environment it has 45 degrees deviation value and in mac-

ro cellular environment, it has 5-10 degrees variation. [2]

2.5.3 Fast fading and Slow fading

Different replicas of the transmitted signal arrive at the receiver due to reflection and

diffraction. These replicated signals vary in amplitude and phase than original one. At

the receiver side, the total received signal is achieved by the superposition/combination

of these replicated signals. The received signal can be constructive if the phase is same

otherwise it is destructive.

9

As the mobile station moves, the amplitude and phase change of the replicated signals

change very quickly as a result the total received signal also change very fast. This phe-

nomenon of rapid fluctuation of amplitude and phase is fast fading. Fast fading is shown

in Figure 2.3 with solid lines.

Figure 2.3: Slow fading and fast fading [2]

The distribution of fast fading varies according to the LOS (Line-of-Sight) and NLOS

(Non-Line-of-Sight) environment between transmitter and receiver. In NLOS, there

exists no single dominating path. This results in random uniformly distributed phase of

all multipath components and the amplitude of received signal is Rayleigh distributed.

The fading in NLOS condition is Rayleigh fading. In LOS condition, the amplitude of

direct signal always has higher amplitude than the others. When a direct path exists, the

total signal amplitude is Rician distributed and the fading caused by LOS is Rician fad-

ing.

Slow fading is a slow variation of the received signal level. It is defined as the variation

of the local mean value of the fast fading over a wide area. It is due to shadow effect

caused large buildings, hills and trees between the transmitter and receiver. These ob-

stacles blocks the main direct path as a result the propagation takes place only from re-

flection and diffraction. Therefore the change in received signal power can be modelled

using log-normal distribution. A dash line in Figure 2.3 shows the slow variation of re-

ceived signal amplitude over a wide area. [2]

2.5.4 Propagation Slope

Propagation slope is the attenuation of the radio wave with respect to the distance in

dB/decade. The propagation slope is defined by the propagation exponent denoted by .

The propagation exponent differs with the environment. In free space, 2 , which cor-

responds to 20dB/decade propagation slope. The propagation slope plays an important

role in the estimation of path loss denoted by L and can be calculated by the equation,

10

0L L d

(2.6)

Distance

Slow fading

Fast fading

Am

pli

tud

e

10

where 0L is the path loss at the reference point, d is the distance between the transmit-

ter and receiver and is the propagation exponent.

The variation of radio condition between the base station and mobile station varies the

propagation slope all the time. The distance where the propagation slope change is

breakpoint distance, B which is calculated by using the equation [2]

4 BTS MSh h

B (2.7)

where BTSh is the height of the base station antenna, MSh is the height of the mobile sta-

tion antenna and is the wavelength of the received signal.

2.5.5 Characteristics of Radio Propagation Environments

The characteristics of radio propagation environments for Global System for Mobile

Communication (GSM) 900 MHz system are summarised in Table 2.1

Table 2.1: Radio channel characteristics for different environment at 900MHz [2]

Environment

type

Angular

spread(degree)

RMS Delay

spread (s)

Fast fading Slow fad-

ing

standard

deviation

(dB)

Propagation

slope

(dB/dec)

Macrocellular

Urban 5-10 0.5 NLOS 7-8 40

Suburban 5-10 NLOS 7-8 30

Rural 5 0.1 (N)LOS 7-8 25

Hilly rural 3 (N)LOS 7-8 25

Microcellular 40-90

11

Empirical models are mainly for macrocellular environment based on extensive meas-

urement campaigns. They are accurate in environments with same characteristics de-

fined by the land use, not by reflections and diffractions. The environment is specified

as urban, suburban, rural and open areas. These models are simple and require low

computational time but they have accuracy problems. Different environment requires

tuning to use empirical model. Okamura-Hata is the most accurate empirical model

which is formulated as

10 10 10 10log 13.82log 6.55log logbs ms bs mL A B f h a h C h d C (2.8)

where

L Path loss [dB]

A Constant (see Table 2.2)

B Constant (see Table 2.2)

f Frequency [MHz] (150 2000MHz f MHz )

bsh Height of the base station antenna [m] 30 200bsm h m

msh Height of the mobile station antenna [m] 1 10msm h m

C Propagation slope

d Distance between mobile station and base station [km] 1 20Km d Km

mC Area type correction factor

The constant parameters A and B differs with respect to frequency and is given as:

Table 2.2: Value of A and B [2]

Parameters Frequency

150-1500MHz 1500-2000MHz

A 69.55 46.3

B 26.16 33.9

Depending upon the size of the city, msa h can be formulated as

For small and medium city,

10 101.1log 0.7 1.56log 0.8ms msa h f h f (2.9) For large city,

2

103.2 log 11.75 4.97ms msa h h (2.10)

In Equation 2.8 the area correction factor varies typically from -3dB (water) and up to

30dB (buildings).

Physical or semi-empirical models are suitable for macro and micro cells are completely

based on geometry of the buildings. They are more accurate than empirical models but

12

require more precise description of the environment and more computation time. COST-

231-Walfisch-Ikegami is the famous semi-empirical model.

Deterministic models are based on analytical estimation of the electromagnetic waves

equation or using ray optical methods. They are mainly for microcellular and picocellu-

lar environment which produces very accurate results in high computation time. These

models rely on accurate 3D building information and material information.

Indoor models are based on the layout of the buildings and building materials. This is

because of the propagation due to reflection, refraction and diffraction of the radio

waves caused by the walls, windows and doors inside the buildings.

2.6 Multiple Access Schemes

Access techniques allow multiple users to share the limited amount of radio spectrum at

the same time. Time Division Multiple Access (TDMA), Frequency Division Multiple

Access (FDMA) and Code Division Multiple Access (CDMA) are the three major ac-

cessing schemes used in a wireless communication system are shown in Figure 2.4.

Figure 2.4: Multiple access techniques: (a) FDMA, (b) TDMA and (c) CDMA [3]

Frequency Channel 4

Frequency Channel 1

Frequency Channel 2

Frequency Channel 3

T

i

m

e

S

l

o

t

1

1

1

T

i

m

e

S

l

o

t

2

T

i

m

e

S

l

o

t

3

T

i

m

e

S

l

o

t

4

T

i

m

e

S

l

o

t

5

5

Guard Band

Guard Band

Guard Band

Guard Band

Gu

ard T

ime

Gu

ard T

ime

Gu

ard T

ime

Gu

ard T

ime

Gu

ard T

ime

Freq

uen

cy

Freq

uen

cy

Time Time (a) (b)

Time

Frequency

Code

(c)

13

In TDMA technique, each user is allocated with unique time slot to access a single radio

channel. These time slots are separated by the guard slots. Data in this scheme is trans-

mitted in the form of burst and hence synchronization is needed. In FDMA technique

the available radio spectrum is divided into large number of narrowband channels. Each

user is allocated with fixed channels and is retained until it is released. These narrow

band channels are separated by guard bands. In this scheme, an unused channel in idle

mode and uneven distribution of the traffic lead towards the wastage of the resources.

The combination of TDMA and FDMA schemes are used in GSM system. In CDMA

technique, the narrowband message signal is multiplied by the spreading signal to pro-

duce a wideband signal. These spreading signals are the sequence of pseudorandom

code which has a chip rate higher than the data rate of the message signal. Each user is

allocated with unique pseudorandom code and they are orthogonal to each other.

14

3. HISTORY AND LTE OVERVIEW

This chapter presents a short history on the development of mobile networks and the

communication methods used back to ages till present. An introduction on today 3G and

4G networks and their evolution path from the previous legacy networks is also ex-

plained in detailed.

3.1 History of Mobile Networks

Going back to the ages, people have their own way to communicate. During those days

people used flag as a medium to convey the information. Later on around 150 BC Greek

people start to use smoke signals for communication. Then after around 1794 Claude

Chappe invented optical telegraph for communication purpose. This is regarded as '' The

Mother of all Networks''. This method was limited by the geography and weather condi-

tions. After that to overcome the limitation, electromagnetic waves were discovered. In

1857 Clark Maxwell derived a theory on electromagnetic fields and introduced wave

equations. Later in 1888 Heinrich Hertz demonstrate the first experiment in Germany on

wave characteristics in space. Based on the Maxwell equations Guglielmo Marconi

demonstrates on wireless telegraphy based on radio waves in 1901. He used huge

transmitter's stations with very high antennas. This system requires high power for

transmission. In 1920 Marconi discovered the short waves which overcome the re-

quirement of huge transmitters and receivers. During this year the technology was in-

stalled in police cars for one way communication. As time passed, radio telephony was

used for military purpose during Second World War. In 1940's hand held transmitters

were available which were bulky and consumes high power. During 1946 US engineers

from Bell lab develop a system which allow user to transmit and receive from automo-

biles. Soon after, American Telephone and Telegraph (AT&T) launched Mobile Tele-

phone service in urban areas with limited coverage. [4]

3.1.1 Evolution towards 1G

In 1970s the modern cellular network were launched. The system utilizes Analog cir-

cuit- switched mainly designed for voice using Frequency Division Multiple Access

(FDMA) technology. This was called First Generation (1G) network. These analog sys-

tems were developed in different parts of the world using Advanced Mobile Phone Sys-

tem (AMPS) in America, Total Access Communication Systems (TACS) and Nordic

Mobile Telephone System (NMT) in Europe and Japanese Total Access Communica-

15

tion Systems (J-TACS) in Japan. In 1979 Nippon Telephone and Telephone Company

(NTT) launched commercially so called 1G network in metropolitan areas of Japan.

This analog system worked on 800-900 MHz frequency bands and used frequency

modulation to transmit the signals. This system was designed to support more number

of users and support user mobility. These analog supporting devices were lighter than

previous development. The main drawbacks of this system are the lack of security and

short battery life. [5]

In 1980s a packet switched technology called X.25 was deployed for data networks

whereas voice networks deployed circuit switched technology. During this stage the

networks for voice and data were completely separated.

In the 90s a new circuit based telephony Integrated Service Digital Network (ISDN)

was introduced to replace the circuit switched analog technology by the digital lines.

This standard helps to transmit voice and the limited data simultaneously. Later, a new

technology Asynchronous Transfer Mode (ATM) was introduced to support data, voice

and video signals to overcome the limitation of X.25. But due to the expensive of the

ATM switches, scientists develop the new technology standard called frame relay. This

standard was simple and support voice and data. [6]


A new modern digital technology called Global System for Mobile (GSM) was intro-

duced in Finland in 1991. The other systems were Personal Digital Cellular Technology

(PDC), integrated Digital Enhanced Network (iDEN), Interim-standard (IS-95) based on

CDMA, IS-136 based on TDMA. They were regarded as the Second Generation (2G)

digital cellular network. Among those systems GSM and IS-95 standard were much

popular. All the limitations of the 1st generation were overcome by this technology as a

result the GSM was spread worldwide. It provides the roaming facilities across the car-

riers. Later, a new packet switched technology was launched to transfer data as short

messaging service with a speed of 9.6 kbps. The use of this technology gave rise to the

use of internet and its protocols. It is designed to operate in 900 MHz and 1800 MHz

frequency band in Europe. The devices were much lighter and cheaper due to digital

technique implementation. The battery life was improved; encryption technique was

implemented for security purpose and more immune to noise. In late 1990s, general

packet radio service (GPRS) was introduced to support high data packets to the existing

GSM networks supporting speed up to 114 kbps. After that Enhanced Data rates for

GSM Evolution (EDGE) come on exists. It uses 8 phase shift keying (8-PSK) modula-

tion technique and combines with GPRS to support the data rate of 200kbps. [5] [6]


During 2000, data services were in high demand. To support this growing, the third

generation mobile communication systems were introduced. This 3G technology sup-

16

ports the high speed internet browsing from the mobile devices. This system also sup-

ports the additional features like video streaming, TV streaming, navigation and multi-

media support. This technology used packet switching for data transmission. Under In-

ternational Telecommunication Union project IMT-2000; a set of 3G standards were

developed such as Universal Mobile Telecommunication System (UMTS), CDMA

2000, Digital Enhanced Cordless Telephone (DECT) and EDGE. The 400MHz to 3GHz

frequency band was allocated for 3G communication systems. In 2001 NTT DoCoMo

launched 3G in Japan based on WCDMA standard. This technology utilizes frequency

reuse concept. In late 2001, UMTS technology was launched commercially in Europe. It

was based on CDMA concept which offers different data rates providing up to 144 kbps

for moving vehicles, up to 384 kbps for pedestrian users and up to 2 Mbps for stationary

users. Due to this technique global roaming and internet connection from any location

was possible. [5]

In 2002, the SK Telecom from South Korea introduced another 3G technology 1xEV-

DO based on CDMA. On the same year the Monet Mobile Network from America

launch 1xEV-DO based on CDMA 2000 standards. Later new protocol standards High

Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access

(HSUPA) were introduced in Release 5 and Release 6 termed in WCDMA evolution to

improve the data transmission rate in mobile communication for downlink and uplink. It

utilizes different modulation and coding techniques. HSDPA offers data rate up to 14.4

Mbps in downlink whereas HSUPA offers 5.76 Mbps in uplink direction. On further

development HSPA+ was introduced in Release 7 to offer higher data traffic up to 84

Mbps in downlink and 22 Mbps uplink direction. [7]


LTE is the next evolution from 3G in wireless mobile networks to 4G introduced in

Release 8. It is IP based technology with flat architecture providing data rate of

100Mbps in downlink and 50Mbps in uplink direction. Low latency, seamless mobility

and efficient use of radio resources are the characteristics of LTE. This emerging tech-

nique can work with previous 2G and 3G networks. LTE-Advanced is another upgrade

evolution from LTE. It is defined in Release 10. It mainly focuses on higher capacity. It

supports multiple antenna systems and Carrier Aggregation (CA) concept.

3.2 Overview of UMTS System

The Universal Mobile Telecommunication System (UMTS) is the 3G cellular technolo-

gy standardized by the 3GPP in Released 99 for voice and data. It uses WCDMA tech-

nique to offer high spectral efficiency and bandwidth. It offers data rate up to 2 Mbps,

seamless handover to GSM/GPRS and high quality speech.

17

3.2.1 UMTS Network Architecture

The UMTS network architecture is classified into three major domains: User Equipment

(UE), UMTS Terrestrial Access Network (UTRAN) and Core Network (CN). Figure 3.1

shows the complete view of the UMTS network architecture with associated elements.

3.2.1.1 User Equipment (UE)

It is the user end device which contains UMTS Subscriber Identity Module (USIM) and

Mobile Equipment (ME). Both are interconnected with each other by Cu interface. The

USIM is a smartcard and it contains the subscriber information, authentication and en-

cryption keys. The ME is used to terminate the radio connection with the network.

Figure 3.1: UMTS network architecture [8]

3.2.1.2 UMTS Terrestrial Access Network (UTRAN)

The UTRAN is similar like Base Station Subsystem (BSS) in GSM technology. It com-

prises of two elements: NodeB and the Radio Network Controller (RNC). The UTRAN

is connected with the UE via Uu interface for communication.

NodeB: It is a base station responsible for handling the user physical data and signal-

ling between the UE and RNC. It performs the power control and load control mecha-

nism. Channel coding, error handling and modulation and demodulation are other major

functions of NodeB.

Radio Network Controller (RNC): It is equivalent to Base Station Controller (BSC) of

GSM. It controls and serves the NodeBs. It is mainly responsible for radio resource

management and controlling the radio channels. It also takes part in routing and switch-

ing the calls to gateway MSC (GMSC). It is interconnect with NodeB via lub interface.

Each RNC are interconnected with lur interface.

CN VLR

PSTN

GMSC Uu MSC

lucs Au

C lub EIR HLR Node B USIM

Cu GR

Gn lups

ME Node B RNC

Node B UE

GGSN

Gi SGSN

Internet

UTRAN

CN

18

3.2.1.3 Core Network (CN)

Its performance is similar to the Network Switching Subsystem (NSS) of 2G system.

The core network is divided into three different categories according to the data they

carried.

Circuit switched elements: It comprise of Mobile Switching Centre (MSC) and

Gateway MSC. The primary function of MSC is to route the voice traffic and

data traffic as well as other value added services. It is also responsible for end to

end connectivity between the users, handling the user mobility and charging. A

database known as Visitor Location Register (VLR) is also included in the MSC.

This data base stores the information of active users connected to the network.

The Gateway MSC forms a gateway to connect UMTS core network with exter-

nal circuit switch network like PSTN.

Packet switched elements: The key elements are Serving GPRS Support Node

(SGSN) and Gateway GPRS Support Node (GGSN). The SGSN is responsible

for carrying and delivering the data packets from one user to another user. It also

takes part in mobility management, logical management and authentication and

billing. The GGSN forms the gateway to connect the GPRS network with exter-

nal packet switched networks like internet.

Shared elements: This section comprises Home Location Register (HLR),

Equipment Identity Register (EIR) and Authentication Centre (AuC). The HLR

is a data base which contains the information of registered users to the network

i.e. all active as well as non active users. The EIR is responsible to decide

whether UE is valid and authorized to access the network or not. Each UE has its

unique International Mobile Equipment Identity (IMEI) code to check the validi-

ty of the device. The AuC is a data base which stores the cipher text of each user

for security purpose. [8]

3.2.2 UMTS Physical, Transport and Logical channels

UMTS channels are classified into three categories: physical, transport and logical

channels. [8]

Table 3.1: UMTS physical channels and their descriptions

Channels Descriptions

Common Control Physical

Channel (CCPCH)

It broadcast the system information in Broadcast Channel

(BCH) and paging information in Paging channel (PCH)

Common Pilot Channel

(CPCH)

It is responsible to identify the scrambling code for syn-

chronization between UE and NodeB

Physical Random Access

Channel (PRACH)

It is responsible to send random access message for syn-

chronization between UE and NodeB

Physical Downlink Shared

Channel (PDSCH)

It is used to share the control information among the UEs

within the NodeB coverage

19

Table 3.1 shows physical channels which are responsible to exchange the information

between the user and UMTS Terrestrial Area Network (UTRAN) in UMTS. In UMTS,

common control physical channel is classified as Primary CCPCH (P-CCPCH) and

Secondary CCPCH (S-CCPCH). Primary common control channel is responsible to

carry system information in Broadcast Channel (BCH) and secondary common control

channel is responsible to carry paging information in transport Paging Channel (PCH).

Table 3.2: UMTS transport channels and their descriptions


Broadcast Control Channel

(BCCH)

This channel is responsible to broadcast the cell infor-

mation in downlink direction

Paging Control Channel

(PCCH)

Its main task is to transmit paging message in downlink

direction

Dedicated Control Channel

(DCCH)

It is used to carry control information to particular UEs in

uplink and downlink direction

Common Control Channel

(CCCH)

It transfers control information in both directions

Dedicated Traffic Channel

(DTCH)

It a channel used to carry user data in uplink and down-

link directions

Common Traffic Channel

(CTCH)

It is used to brocast the data to a group of UEs in down-

link direction

Table 3.2 shows transport channels that describe the characteristics of the data trans-

ferred on the physical layer. The transport layers are interface between physical and

MAC layer.

Table 3.3: UMTS logical channels and their descriptions


Synchronization Channel

(SCH)

It is responsible for synchronization between the UEs and

NodeBs

Dedicated Transport Chan-

nel (DCH)

It is responsible to transport the data to the particular UE

in both uplink and downlink directions

Broadcast Channel (BCH) It broadcasts the information necessary to UE to identify

the cell and the network

Forward Access Channel

(FACH)

It mainly transmits the control data or user data to UE in

the downlink direction

Paging Channel (PCH) It carries paging information to establish the connection

with UE in the downlink direction

Random Access Channel

(RACH)

It is responsible to carry service request from UEs in up-

link direction

Downlink Shared Channel

(DCH)

This channel is shared by different UEs to transmit con-

trol information or user information in downlink

20

Table 3.3 presents logical channels which are interface in between Medium Access

Control (MAC) layer and Radio Link Control (RLC) layer whereas. Logical channels

are mapped into transport channel in the Media Access Control (MAC) layer. They de-

scribe what type of data is transferred between the user and the network.

3.3 High Speed Downlink Packet Data Access (HSDPA)

High Speed Packet Access (HSPA) introduced by 3GPP is evolved from WCDMA

network to support the high data rate, reduce latency and increased capacity. Initially

High Speed Downlink Packet Data Access (HSDPA) was introduced in Release 5 and

later on High Speed Uplink Packet Data Access (HSUPA) was introduced in Release 6.

HSDPA protocol supports high speed data in the downlink direction.

A new transport channel High Speed Downlink Shared Channel (HS-DSCH) is added to

WCDMA for HSDPA for faster downloads in downlink directions. This channel ena-

bles to allocate a fraction of radio resources to a specific user for data transmission.

HSDPA improves the data rate by a factor of 5 as compare to the WCDMA. Theoreti-

cally, it gives the data rate of 8-10 Mbps and even more with Multiple Input Multiple

Output (MIMO) technique. This technology can be implemented on 5 MHz channels

available for UMTS system. [8] [9]

3.4 LTE Evolution and Upgrade Path

LTE was introduced by 3GPP to overcome the limitations of the previous network tech-

nology in terms of system coverage, performance and capacity. This new technology

provide high data rates, supports new features like video chatting and multimedia ser-

vices and serve the existing terminals at the same time in efficient manner. The 3GPP

emphasized the key features and performance requirement targets for LTE. [10] [11]

Higher peak data rate of 100 Mbps for downlink and peak 50 Mbps for uplink in

20 MHz band. It offers downlink data rate of 150 Mbps in downlink using 2 2

MIMO and 75 Mbps in uplink using 1 2 antenna configuration for 20MHz

channel.

Significantly improved spectral efficiency, 2 4 times higher than that of 3GPP

Release 6 standards. Peak spectral efficiency is 5 bps/Hz in downlink and 2.5

bps/Hz in uplink is achieved with two receive antennas and one transmit antenna

configuration.

Interoperability with previous 3G or 2G systems and other non-3GPP technolo-

gies. It is optimized for low speed ( 0 15 /km h ) mobile terminals and support

high speed 120 / 350 /km h km h mobile terminals or even higher up to

500 /km h depending upon frequency band.

Provides high quality of service. It has radio access network latency less than 10

ms. The control plane latency from idle mode to active mode is less than 100 ms

21

while for user plane latency is less than 50 ms for real time application and

voice.

Bandwidth flexibility allocation with 1.4, 3, 5, 10, 15 and 20 MHz

With growing demand of new technologies to support high data rates and minimum

latency with low cost and higher performance, evolution is always needed. These de-

mands are always taken into account during evolution on cellular network for users.

LTE is the new emerging technology evolved from 2G and 3G standards like Global

System for Mobile Communication (GSM), Universal Mobile Telecommunication Sys-

tem (UMTS) and High Speed Packet Data (HSPA). Figure 3.2 shows the LTE evolution

path from its predecessor networks based on release dates.

Figure3.2: 3GPP LTE Evolution path [10]

Figure 3.3 shows the LTE upgrade path for 3GPP and Non-3GPP technologies.

Figure3.3: LTE upgrade path for 3GPP and Non-3GPP technologies [8] [10]

Any 2G and 3G network can be upgraded directly towards LTE by any vendors and

operators without following any specific path. This upgrade path helps to reduce cost

while adopting new technologies and standards.

3.5 LTE Network Architecture

Figure 3.4 shows the LTE network architecture and the elements associated with it. This

architecture is mainly designed to support the IP system. The architecture was made flat

WCDMA HSDPA HSUPA HSPA Evolution

LTE LTE Advanced

R99 Rel 4 Rel 5 Rel 6 Rel 7 Rel 8 Rel 10

GSM EDGE WCDMA HSPA LTE

Non-3GPP Technologies

22

to reduce the number of network elements. This helps to increase the system perfor-

mance. The User Equipment (UE), Evolved Universal Terrestrial Radio Access Net-

work (EUTRAN), Evolved Packet Core (EPC) and Service layer are the major logical

elements in LTE which are briefly described below. [11]

Figure 3.4: LTE system architecture [10]

3.5.1 User Equipment (UE)

User Equipment are hand held devices for user. It may be a smart phone, dongle device

or a laptop used to establish, maintain and remove the radio connectivity according to

user requirement. Each UE has its own Universal Subscriber Identity Module (USIM) to

separate and authenticate from other UEs. They are connected with EUTRAN with

LTE-Uu interface. [12]

3.5.2 Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)

The EUTRAN is the evolution from UTRAN network which contains only one node

called eNodeB. The NodeB and Radio Network Controller (RNC) are combined to form

a single eNodeB to flatten the network architecture in LTE. This reduces the network

latency. The eNodeB performs the function of both NodeB and RNC. Each eNodeB are

interconnected with each other via X2 interface. The EUTRAN also interconnects MME

and SGW with S1-MME interface and S1-U interface respectively. Figure 2.4 also

shows the EUTRAN interface with other network elements. These interfaces are stand-

ard and defined by 3GPP to facilitate the multi-vendors.

The main functions of EUTRAN are the physical layer processing, Radio Resource

Management (RRM) and Mobility Management. It performs power control of the sig-

nal, analog-digital-analog conversion of the signal at the air interface, encryption and

decryption of user plane data as well as compression of IP header to reduce redundancy.

The EUTRAN also manage the radio resources. It indicates the type of modulation

used, retransmission of the data, Quality of Service (QoS) management, resource sched-

PCRF

P-GW

HSS

S-GW

MME

UE

eNodeB

eNodeB

E-UTRAN

EPC

LTE-Uu

S1-U

S1-MME

Gxe

S5/S8

S11 S6a

Gx

23

uling etc. It receives measurement report of RSRP and RSRQ value from UE for hand-

over decisions.

3.5.3 Evolved Packet Core (EPC)

EPC is the core of the network. The architecture is evolved from the GPRS architecture

which is completely based on end to end IP connectivity. The main functions of EPC

are to support seamless mobility, QoS and charging functions of IP based networks.

The major elements of EPC discussed below.

3.5.3.1 Mobile Management Entity (MME)

MME is a control node in LTE network. It is responsible for mobility management,

tracking the location as well as security functions between the UE and the network. It is

linked with Home Subscription Server (HSS) via S6a interface for authentication and

authorization of the users. It performs activation and deactivation of the radio bearers

with Serving Gateway (SGW) connected with S11 interface. The MME also involved in

management of Non-access stratum (NAS) signalling security. At the radio interface the

NAS forms the apical stratum of the control plane between UE and MME. Its task is to

support user mobility and authentication and session management for IP connection

between UE and Packet Gateway (PDN- GW). The MME is also responsible for hando-

ver performance in between the LTE and previous legacy 3G/2G networks via S3 inter-

face (SGSN to MME). It performs selection of MME and Serving GPRS Support Node

(SGSN) for handover change. [10]

3.5.3.2 Serving Gateway (S-GW)

The S-GW is a user plane element for forwarding and receiving the IP packets from UE

to P-GW and vice versa. It acts as a local or mobility anchor during handover process.

The S-GW switched the user plane path from serving eNodeB to a new eNodeB when

MME sends the request depending upon the UE mobility. The IP packets received from

the eNodeB are forwarded to the S-GW using GPRS Tunnelling Protocol (GTP). The S-

GW receives the IP packets and forward to the Packet Gateway (PDN-GW) using the

same GTP. The same thing happens in reverse way. As in case when a UE goes to idle

mode all of sudden while receiving the data packets from PGW, the SGW resumes the

data flow towards the MME and holds all the incoming packets in its buffer container.

Meanwhile it request MME to send a paging message to that idle UE. Once UE in con-

nected mode, the buffer data packets are forwarded towards UE along with the new

packets from the PGW. [10]

3.5.3.3 Packet Gateway (P-GW)

The P-GW is the gateway to the external network. The working function is somewhat

similar with GPRS Support Node (GGSN) in UMTS system. Internet, IP Multimedia

Subsystem (IMS) and other service type networks are the example of external packet

data networks. All these packet data networks are identified within the LTE networks by

24

their own Access Point Name (APN). APN is needed when a UE wants the Internet ser-

vice to be connected. The PDN gateway assigns the temporary address to each connect-

ed UE based on IPv4 or IPv6 standard. The PDN-GW can act as an IP anchor as it al-

lows UE to move from eNodeB to eNodeB from serving SGW to another new SGW. It

is connected with PCRF via S7 interface. PCRF is responsible to set the quality of ser-

vice (QoS) for each user whereas PGW enforces the QoS policy. [10]

3.5.3.4 Home Subscription Server (HSS)

The HSS is a database which contains the user information. It includes the subscriber

information International Mobile Subscriber Identity (IMSI) and Mobile Station Inter-

national Subscriber Directory Number (MSISDN) value used for user identification and

addressing. It also stores user subscription state user quality of information. The Au-

thentication Centre (AC) is integrated with HSS responsible for authentication, cipher-

ing and integration of radio path.

25

4. LTE RADIO INTERFACE

The 3GPP technologies are designed for smooth inter-working and coexistence. This

chapter begins with a brief description about air interface technologies for LTE. It also

deals with framing structure and interfaces used in LTE technology. Different physical,

logical, control channels and reference signals used in LTE are also explained. A short

introduction on different Medium Access Control (MAC) layer and physical layer func-

tions are discussed at the end of this chapter.

4.1 Air Interface Technologies for LTE

LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) channel access-

ing schemes for downlink and Single Carrier-Frequency Division Multiple Access (SC-

FDMA) for uplink purpose. These two schemes provide mutual orthogonality between

the users reducing the interference and increasing capacity of the network by utilizing

the available spectrum in efficient manner.

4.1.1 OFDMA for Downlink Transmission

In any communication system, different modulation schemes and multiple access tech-

niques are used to increase the system capacity and system performance. Multiple ac-

cess techniques are used to limit bandwidth and increase number of channels so that it

can be shared among users efficiently. In LTE, OFDMA technology is used in downlink

direction as it meets all the specification specified in 3GPP.

Orthogonal Frequency Division Multiplex (OFDM) is a type of multicarrier technology

which subdivides the available spectral bandwidth into a number of closely spaced nar-

rowband subcarriers. These subcarriers are made mutually orthogonal so they can be

overlapped with each other to provide high spectral efficiency and also to mitigate the

interference between channels. In order to achieve orthogonality, carrier frequencies are

made equal to the reciprocal of the symbol period. For a single carrier sampling instant,

the other carriers have zero value at that frequency. Figure 4.2 is a complete block dia-

gram of OFDM transmitter and receiver. [10]

Figure 4.1 shows the orthogonality of subcarriers. OFDM can be used in both Time Di-

vision Duplexing (TDD) and Frequency Division Duplexing (FDD) modes. Quadrature

Phase Shifting Keying (QPSK), 16 Quadrature Amplitude modulation (16QAM) and 64

Quadrature Amplitude Modulation (64QAM) are the different modulation types that can

be used for OFDM signal. In OFDM each user is allocated with fixed bandwidth for

specific time.

26

Figure 4.1: Equally spaced OFDM sub-carriers [10]

Orthogonal Frequency Division Multiple Access (OFDMA) use OFDM in which multi-

ple users are allocated to different subcarriers. These subcarriers are created by the In-

verse Fast Fourier Transform (IFFT) transformation of the signal at the transmission

side. The subcarriers are spaced 15 kHz apart from each other which gives symbol rate

1 / 15 kHz = 66.7 s. This principle allocates each user by a resource block equivalent

to 12 sub-carriers in frequency and 1 Transmission Time Interval (1TTI) corresponds to

1ms in time. A Cyclic Prefix (CP) known as guard interval is added to the transmitted

symbols. It is done to remove an ISI caused by multipath components of the transmitted

symbols. Figure 4.2 shows the cyclic extension added after the IFFT block. The cyclic

extension is nothing but a copied tail part of symbol. It is attached to the beginning of

the symbols before transmission to separate the symbols with a time interval to neutral-

ize the delay spread caused by the multipath fading. At the receiver end the cyclic prefix

is removed with the FFT operation to extract the correct sent bits. There are two differ-

ent type of CP: normal with 4.67 s and extended with 16.67 s. The extended CP is

mainly used for high delay spread environment while normal CP is used for other envi-

ronment. [10]

Figure 4.2: OFDM transmitter and receiver [10]

The length of CP is important as it affect on the data rates of any system. Increase in CP

increases the timing gap between two frames and this needs extra time to receive the

signal form multipath channel. So, higher CP length leads to low data rates. So the dura-

Modulator Serial to

parallel

IFFT Cyclic

Extension

Remove

cyclic Ex-

tension

Serial to

parallel FFT Equi-

lizer Demodulator

F

Bandwidth

Bits

Bits

Transmitter side

Receiver side

15 KHz

Bandwidth

27

tion must be small in comparison with multipath channel duration. Figure 4.3 (a) is an

example of subcarrier allocation in OFDMA systems.

Figure 4.3: Sub-carrier allocation: (a) OFDMA and (b) SC-FDMA

4.1.2 SC-FDMA for Uplink Transmission

Peak-to-Average Power Ratio (PAPR) is a major problem in Orthogonal Frequency

Division Multiple Access (OFDMA). It is caused by the interference of the subcarrier

signals. Due to interference some of the transmitted signals have peak value larger than

the typical one. It leads to the requirement of linear circuits with a large dynamic range

otherwise clipping of the signals take place which results in distortion, out of band radi-

ation of the transmitted signal and low efficiency. Therefore, in uplink transmission

OFDMA is not used because of high PAPR value. Due to high PAPR, RF power ampli-

fier within a mobile is unable to perform efficiently. So a high linear RF power is re-

quired. This decreased the battery life. So, another type of multiple access scheme

called Single Carrier- Frequency Division Multiple Access (SC-FDMA) is introduced in

uplink transmission which keeps the advantage of Orthogonal Frequency Division Mul-

tiplexing (OFDM) as multicarrier transmission and high utilization of bandwidth. It has

low PAPR value as well. [10] [11]

SC-FDMA utilizes the single carrier modulation scheme. It is also known as OFDM

system with a Discrete Fourier Transform (DFT) mapped because frequency domain

symbols are generated using DFT. Thus generated symbols are mapped to available

subcarriers by sub-carrier mapping techniques and after that Inverse Fast Fourier trans-

form (IFFT) is performed as like in OFDMA. There are two types of sub carrier map-

ping: localized and distributed. In localized mapping modulated symbols are assigned to

adjacent subcarrier whereas in distributed mapping modulated symbols are equally

spaced over the entire bandwidth.

Figure 4.4 shows the block diagram of SC-FDMA transmitter and receiver. In OFDMA

each subcarrier carries the information of one specific transmitted symbol whereas in

SC-FDMA each sub-carrier carries the information of all transmitted symbols. Symbols

in SC-FDMA are transmitted in serial manner. The arrangement of subcarriers in SC-

FDMA technology is shown in above Figure 4.3(b).

F1 F2 F3 F4 F5 Fn

Frequency

F1 F2 F3 F4 F5 Fn

Frequency (a)

(b)

28

Figure 4.4: Block diagram of SC-FDMA transmitter and receiver [10]

4.1.3 Multiple Antenna Technology

The received signal level from transmitter is always affected by the natural obstacles or

manmade obstacles present in the environment. Obstacles like buildings, trees, moun-

tains cause fading due to the multipath or shadow effect from it. Diversity technique can

overcome fading problem. Diversity technique produces uncorrelated radio channels

between the transmitter and receiver. Figure 4.5 shows different types of diversity tech-

niques like Single Input Single Output (SISO), Single Input Multiple Output (SIMO),

Multiple Input Single Output (MISO) and Multiple Input Multiple Output (MIMO)

available in the wireless communication.

Figure 4.5: Diversity techniques [13]

The LTE network studied during thesis did not use MIMO technology. So this topic is

not discussed in detail.

Tx Rx Tx

Rx

Rx0

Rx1

Tx0

Tx1

Tx0

Tx1

Rx0

Rx1

SISO

SIMO

MISO MIMO

Modulator DFT IFFT

Sub-

carrier

mapping

Remove

cyclic

Extension

MMSE

Equali-

zer

FFT IDFT Demodulator

Cyclic

Extension

F

Receiver side

Transmitter side

Bits

Bits Bandwidth

29

4.2 LTE Framing Structure

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